Modified Carrier-Overlapped PWM With Balanced Capacitors and Eliminated Dead-Time Spikes for Four-Level NNPC Converters Under Low Frequency

Abstract

Due to the lack of sufficient redundant states, four-level nested neutral-point clamped (4L-NNPC) converters suffer from severe flying capacitor (FC) voltage fluctuations under low fundamental frequencies, which restricts their applications in practice. To deal with this issue, a modified carrier-overlapped pulsewidth modulation (PWM) (COPWM) is proposed in this article, which could achieve zero average current flow through the two FCs over each carrier period. As a result, their voltages can be balanced naturally under steady states theoretically, and the low-frequency FC voltage fluctuation issue would no longer exist. In order to compensate for any nonideal factors in real industrial applications, an active control by dynamically adjusting the correspondence between carriers and switches is further applied for better control performance. A dead-time compensation scheme is proposed accompanied by this modified COPWM to eliminate the unexpected dead-time-induced voltage spikes of 4L-NNPC converters. The effectiveness of the proposed modified COPWM in regulating the FCs under low frequency and eliminating the dead-time-induced voltage spikes is verified by both simulation and experimental results.